Subsets: creators

Extend Subset model with a list of SubsetCreator records (similar to Dataset#creators):

/** The creators. */
@OneToMany(cascade = { CascadeType.ALL }, mappedBy = "subset", orphanRemoval = true, fetch = FetchType.LAZY)
@Field(type = FieldType.Object)
@JsonView({ JsonViews.Public.class })
private List<SubsetCreator> creators;

Add API methods to SubsetController and SubsetService. You can copy most of the business logic from DatasetCreatorServiceImpl.